/Engine/Source/Runtime/ApplicationCore/Public/GenericPlatform/ITextInputMethodSystem.h
Editable texts should implement this class and maintain an object of this type after registering it. Methods of this class are called by the system to query contextual information about the state of the editable text. This information is used by the text input method system to provide appropriate processed input. Methods of this class are also called by the system to provide processed text input.
class ITextInputMethodContext